Hot work tool steel

ABSTRACT

A matrix type hot work tool steel, in use, has an improved abrasive wear resistance in demanding applications. The steel is suited for applications in hot forging, die casting or hot extrusion. It is also suitable for press hardening, in particular for press hardening of Advance High Strength Steel (AHSS) and has a high hot wear resistance. The hot work tool steel has a composition including, in weight % (wt. %): C 0.65-0.85; Si 0.03-0.8; Mn 0.1-1.8; Cr 4.5-6.6; Mo 1.8-3.5; V 1.3-2.3; Al≤0.1; N≤0.12; Ni≤1; W≤0.5; Co≤2; Cu≤1; Nb≤0.1; Ti≤0.05; Zr≤0.05; Ta≤0.05; B≤0.01; Ca≤0.01; Mg≤0.01; REM≤0.2; and balance Fe and impurities.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a National Stage Entry into the United States Patent andTrademark Office from International Patent Application No.PCT/SE2021/050562, filed on Jun. 11, 2021, which relies on and claimspriority to Swedish Patent Application No. SE 2050705-9, filed on Jun.12, 2020, the entire contents of both of which are incorporated hereinby reference.

FIELD OF THE INVENTION

The invention relates to a matrix type hot work tool steel.

BACKGROUND OF THE INVENTION

Vanadium alloyed matrix tool steels have been on the market for decadesand attained a considerable interest because of the fact, that theycombine a high wear resistance with an excellent dimensional stabilityas well as a good toughness. A matrix tool steel is a steel which doesnot contain any primary carbides or only an extremely low content ofsmall primary carbides and which has a matrix consisting of temperedmartensite.

U.S. Pat, No. 3,117,863 is probably the first patent directed to amatrix steel. The basic idea in the U.S. Pat. No. 3,117,863 was tocreate a steel having the composition of the matrix of a known highspeed steel (HSS). The structure of this type of steel was developed inorder to improve the toughness and the fatigue strength of the steel byrefining the microstructure.

WO 03/106727 A1 of the present applicant discloses a hot work matrixsteel having an excellent toughness and ductility as well as a good hotstrength and wear resistance. The material is known in the market underthe name UNIMAX®.

EP1 300 482 A1 discloses another matrix steel having a high hardness andwear resistance in combination with a very high toughness and istherefore particularly suited for tools that are stressed at elevatedtemperatures such as tools for hot and warm forming. This steel is knownin the market under the name W360 ISOBLOC ® and has a nominalcomposition of 0.50% C, 0.20% Si, 0.25% Mn, 4.5% Cr, 3.00% Mo and 0.60%V.

Matrix steels are normally produced by vacuum arc re-melting (VAR) orelectro slag re-melting (ESR) in order to improve the chemicalhomogeneity and the micro-cleanliness. Further examples of hot work toolmatrix steels are given in JP2003226939A, EP3050986A1, US2004/0187972 Aland US2005/0161125A1.

Modern matrix steels are being developed with the aid of software forthe calculation of phase diagrams and equilibrium phase balances as afunction of temperature. Themo-Calc® (TC) is a user-friendly andfrequently used software for this purpose in order to find outcompositions resulting in a large austenitic single phase area atsoaking temperatures, because of the fact that the dissolution ofpossibly existing MC carbides formed by segregation during casting is ofprima importance.

Hot work matrix steels have a wide range of applications such as diecasting and forging. The steels are generally produced by conventionalmetallurgy followed by Electro Slag Remelting (ESR). However, a drawbackof the known steels is the limited wear resistance. In particular, theabrasive wear resistance may limit the life of the known steels indemanding hot work operations such as hot forging, extrusion and presshardening. These tools are expensive and often need to be welded forrepair. Accordingly, the weldability is of importance. However, theweldability of tool steel with high carbon contents is usuallyconsidered to be poor and requiring special measures such as highpreheating temperatures. It would therefore be useful if the steel couldbe welded with standard welding consumables, preferable withoutpreheating.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a matrix type hot worktool steel, in use having an improved abrasive wear resistance indemanding applications. In particular, the steel should be suited forapplications in hot forging, die casting or hot extrusion. It shouldalso be suitable for press hardening, in particular for press hardeningof Advanced High Strength Steel (AHSS). For these applications, the hotwear resistance needs to be high.

The tempering resistance is an important property, because in use thesteel may be subjected to high temperatures for long times. Accordingly,it is preferred that the steel not only has a high hardness afterhardening but also that the hardness decrease is small. Furtherimportant properties include high ductility and toughness, which impliesthat the steel should have a high cleanliness with respect tomicro-slag, a complete freedom from grain boundary carbides as well as auniform hardness for thicknesses up to 300 mm.

It should be possible to adjust the hardness over a large interval inorder to optimize the steel for the intended use. It should also bepossible to obtain a high tensile strength and yield strength incombination with a sufficient ductility.

The foregoing objects, as well as additional advantages are achieved toa significant measure by providing a hot work tool steel having acomposition as set out in the claims.

The invention is defined in the claims.

DETAILED DESCRIPTION OF EMBODIMENT(S) OF THE INVENTION

The importance of the separate elements and their interaction with eachother as well as the limitations of the chemical ingredients of theclaimed alloy are briefly explained in the following. All percentagesfor the chemical composition of the steel are given in weight % (wt. %)throughout the description. The amount of the hard phases is given involume % (vol. %). Upper and lower limits of the individual elements canbe combined freely within the limits set out in the claims. Thearithmetic precision of the numerical values can be increased by one ortwo digits. Hence, a value given as e.g. 0.1% can also be expressed as0.10% or 0.100%.

Carbon (0.5-0.9%)

is to be present in a minimum content of 0.5%, preferably at least 0.55,0.60, 0.66, 0.67, or 0.68%. The upper limit for carbon is 0.9% and maybe set to 0.85, 0.80, 0.75, 0.74, 0.73, or 0.72%. Preferred ranges are0.6-0.8% and 0.65-0.75%. In any case, the amount of carbon should becontrolled such that the amount of primary carbides of the type M₂₃C₆,M₇C₃ and M₆C in the steel is limited, preferably the steel is free fromsuch primary carbides.

Silicon (0.03-0.8%)

Silicon is used for deoxidation. Si is present in the steel in adissolved form. Si is a strong ferrite former and increases the carbonactivity and therefore the risk for the formation of undesired carbides,which negatively affects the impact strength. Silicon is also prone tointerfacial segregation, which may result in decreased toughness andthermal fatigue resistance. Si is therefore limited to 0.8%. The upperlimit may be 0.7, 0.6, 0.5, 0.40, 0.35, 0.30, 0.28, 0.27, 0.26, 0.25,0.24, 0.23 and 0.22%. The lower limit may be 0.05, 0.10, 0.11, 0.12,0.13, 0.14 or 0.15%.

Manganese (0.1-1.8%)

Manganese contributes to improving the hardenability of the steel andtogether with sulphur manganese contributes to improving themachinability by forming manganese sulphides. Manganese shall thereforebe present in a minimum content of 0.1%, preferably at least 0.2, 0.3,0.35 or 0.4%. At higher sulphur contents manganese prevents redbrittleness in the steel. Mn may also cause undesirablemicro-segregation resulting in a banded structure. The steel shallcontain maximum 1.8%, preferably maximum 0.8, 0.75, 0.7, 0.6, 0.55 or0.5%.

Chromium (4.0-6.6%)

Chromium is to be present in a content of at least 4% in order toprovide a good hardenability in larger cross sections during heattreatment. If the chromium content is too high, this may lead to theformation of high-temperature ferrite, which reduces thehot-workability. The lower limit may be 4.5, 4.6, 4.7, 4.8 or 4.9%. Theupper limit may be 6.0, 5.9, 5.8, 5.7, 5.6, 5.5, 5.4, 5.3, 5.2 or 5.1%.

Molybdenum (1.8-3.5%)

Mo is known to have a very favourable effect on the hardenability.Molybdenum is essential for attaining a good secondary hardeningresponse. The minimum content is 1.8%, and may be set to 1.9, 2.0, 2.1,2.15 or 2.2%. Molybdenum is a strong carbide forming element and also astrong ferrite former. The maximum content of molybdenum is therefore3.5%. Mo may be limited to 2.9, 2.7, 2.6, 2.5, 2.4 or 2.3%.

Tungsten (W≤0.5%)

Tungsten is not an essential element in the present invention. The upperlimit is 0.5% may be set to 0.4, 0.3, 0.2 or 0.1%.

Nickel (≤1%)

Nickel is not an essential element in the present invention. The upperlimit may be set to 0.5, 0.4, 0.3 or 0.25%.

Vanadium (1.3-2.3%)

Vanadium forms evenly distributed primary precipitated carbides andcarbonitrides of the type VC and V(C,N) in the matrix of the steel.These carbides and carbonitrides may also be denoted MX, wherein M ismainly V but Cr and Mo may be present and X is one or more of C, N andB. However, in the following only VC will be used with the same meaningas MX. Vanadium is used in order to form a controlled amount ofrelatively large VC and shall therefore be present in an amount of1.3-2.3%. The lower limit may be set to 1.35, 1.4, 1.45, 1.5 or 1.55%.The upper limit may be set to 2.2, 2.1, 2.0, 1.9, 1.8, 1.7 or 1.65%.

Aluminium (≤0.1%)

Aluminium may be used for deoxidation in combination with Si and Mn. Thelower limit is set to 0.001, 0.003, 0.005 or 0.007% in order to ensure agood deoxidation. The upper limit is restricted to 0.1% for avoidingprecipitation of undesired phases such as A1N. The upper limit may be0.05, 0.04 or 0.3%.

Nitrogen (≤0.12%)

Nitrogen is an optional element. N is restricted 0.12% in order to avoidtoo high an amount of hard phases, in particular V(C,N). However, thenitrogen content may be balanced against the vanadium content in orderto form primarily precipitated vanadium rich carbonitrides. These willpartly be dissolved during the austenitizing step and then precipitatedduring the tempering step as particles of nanometer size. The thermalstability of vanadium carbonitrides is considered to be better than thatof vanadium carbides, hence the tempering resistance of the tool steelmay be improved and the resistance against grain growth at highaustenitizing temperatures may be enhanced. If the nitrogen content isdeliberately controlled for the above reason then the lower limit may beset to 0.006, 0.007, 0.08, 0.09, 0.01, 0.012, 0.013, 0.014 or 0.015%.The upper limit may be 0.11, 0.10, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04 or0.03%.

Copper (≤1%)

Cu is an optional element, which may contribute to increase the hardnessand the corrosion resistance of the steel. However, it is not possibleto extract copper from the steel once it has been added. Thisdrastically makes the scrap handling more difficult. For this reason,copper is normally not deliberately added. The upper limit may berestricted to 0.5, 0.4, 0.3, 0.2 or 0.15%.

Cobalt (≤5%)

Co is an optional element. Co causes the solidus temperature to increaseand therefore provides an opportunity to raise the hardeningtemperature. During austenitization it is therefore possible to dissolvelarger fraction of carbides and thereby enhance the hardenability.However, Co is expensive and a large amount of Co may also result in adecreased toughness and wear resistance. The maximum amount is therefore5%. However, a deliberate addition of Co is generally not made. Themaximum content may be set to 2, 1, 0.5 or 0.2%.

Niobium (≤0.1%)

Niobium is similar to vanadium in that it forms carbonitrides of thetype M(N,C). However, Nb results in a more angular shape of the M(N,C)and may reduce the hardenability at high contents. The maximum amount istherefore 0.1%, preferably 0.05%. Nb precipitates are more stable than Vprecipitates and may therefore be used for grain refinement, since thefine dispersion of NbC plays the role of pinning the grain boundariesleading to grain refinement and improved toughness as well as improvedresistance to softening at high temperatures. For this reason, Nb is anoptional element and may be present in an amount of ≤0.1%. The upperlimit may be set to 0.06, 0.05, 0.04, 0.03 0.01 or 0.005%. The lowerlimit may be set to 0.005, 0.006, 0.007, 0.008, 0.009 or 0.01%.

Ti, Zr and Ta

These elements are carbide formers and may be present in the alloy inthe claimed ranges for altering the composition of the hard phases.However, normally none of these elements are added. The amount of eachelement is preferable ≤0.5%, 0.1% or ≤0.05%, more preferably 0.01% or0.005%.

Boron (≤0.01%)

B may be used in order to further increase the hardness of the steel.The amount is limited to 0.01%, preferably ≤0.006% more preferably0.005%.

Ca, Mg and REM (Rare Earth Metals)

These elements may be added to the steel in the claimed amounts formodifying the non-metallic inclusion and/or in order to further improvethe machinability, hot workability and/or weldability. The amount of Caand Mg is preferably ≤0.01%, more preferably ≤0.005%. The amount of REMis preferably ≤0.2%, more preferably ≤0.1% or even 0.05%.

Impurity Elements

Impurity elements cannot be avoided during the manufacturing of thesteel. Impurity elements are therefore included in the balance and thelevel of said elements is not essential to the definition of the presentinvention.

P, S and O are the main impurities, which generally have a negativeeffect on the mechanical properties of the steel. These elements areunavoidable and may occur in in the steel at common impurity contents.However, since these elements may have a negative effect on theproperties in steel, the impurity contents thereof may be furtherlimited. Preferred limitations are set out as follows. P may be limitedto 0.1, 0.05 or 0.03%. S may be limited to 0.5, 0.1 0.05 0.0015, 0.0010,0.0008, 0.0005 or even 0.0001%. O may be limited to 0.01, 0.003, 0.0015,0.0012, 0.0010, 0.0008, 0.0006 or 0.0005%.

Steel Production

The tool steel having the claimed chemical composition can be producedby conventional metallurgy including melting in an Electric Arc Furnace(EAF) and further refining in a ladle, optionally followed by a vacuumtreatment before casting. The ingots may also be subjected to ElectroSlag Remelting (ESR) in order to further improve the cleanliness and themicrostructural homogeneity of the ingots. In addition the steel mayalso be subjected to Vacuum Induction Melting (VIM) and/or Vacuum ArcRemelting (VAR). An alternate processing route for the claimed steel isgas atomizing followed by hot isostatic pressing (HIP) to form a HIPedingot, which also may be used in the condition as-HIPed. The ingots maybe subjected to further hot working to final dimension as well as tosoft annealing to a Brinell hardness of ≤360 HBW, preferably ≤300 HBW.The Brinell hardness is measured with a 10 mm diameter tungsten carbideball and a load of 3000 kgf (29400N) and may also be denotedHBW_(10/3000). The steel may be subjected to hardening and temperingbefore being used.

The steel is normally delivered to the customer in the soft annealedcondition having a ferritic matrix with an even distribution of carbidestherein. The soft annealed steel has uniform properties also for largedimensions and according to a preferred embodiment the uniformity inhardness should have a mean hardness of ≤360 HBW and for a thickness ofat least 100 mm and the maximum deviation from the mean Brinell hardnessvalue in the thickness direction measured in accordance with ASTM E10-01is less than 10%, preferably less than 5%, and wherein the minimumdistance of the centre of the indentation from the edge of the specimenor edge of another indentation shall be at least two and a half timesthe diameter of the indentation and the maximum distance shall be nomore than 4 times the diameter of the indentation.

The atomized powder may also be used for additive manufacturing.

Hereinafter, the present invention will be described in more detail.

The hot work steel according to the present invention consists of inweight % (wt. %):

C 0.5-0.9 Si 0.03-0.8  Mn 0.1-1.8 Cr 4.0-6.6 Mo 1.8-3.5 V 1.3-2.3 Al≤0.1 N ≤0.12 Ni ≤1 W ≤1 Co ≤5 Cu ≤1 Nb ≤0.1 Ti ≤0.05 Zr ≤0.05 Ta ≤0.05 B≤0.01 Ca ≤0.01 Mg ≤0.01 REM ≤0.2 balance Fe and impurities.

Preferably, the hot work tool steel fulfils at least one of thefollowing requirements:

C 0.6-0.8 Si 0.05-0.6  Mn 0.2-0.8 Cr 4.4-5.6 Mo 2.0-2.5 V 1.5-1.9 Al≤0.05 N ≤0.08 Ni ≤0.5 W ≤0.5 Co ≤2 Cu ≤0.5 Nb ≤0.05 Ti ≤0.01 Zr ≤0.01 Ta≤0.01 B ≤0.006 Ca ≤0.005 Mg ≤0.005 REM ≤0.1

More preferably the composition of the steel fulfils one or more of thefollowing requirements:

C 0.65-0.75 Si 0.15-0.5  Mn 0.4-0.5 Cr 4.9-5.1 Mo 2.2-2.3 V 1.5-1.7 Al≤0.03 N ≤0.05 Ni 0.25 W ≤0.2 Co ≤1 Cu ≤0.2 Nb ≤0.005 Ti ≤0.005 Zr ≤0.005Ta ≤0.005 REM ≤0.05

Preferably the steel fulfils at least one of the following requirements:

C 0.66-0.75 Si 0.15-0.25 V 1.52-1.68 Al 0.001-0.03  N ≤0.05 W ≤0.1 Cu≤0.15

In a particular preferred embodiment all of these requirements arefulfilled.

In order to enhance the resistance against abrasive wear the compositioncan be adjusted such that the steel in the hardened and temperedcondition contains a small and controlled amount of vanadium carbideshaving a size of larger than or equal to 1 μm. The size is given asEquivalent Circular Diameter (ECD), which is calculated from the imagearea (A) obtained in an image analysis. The ECD has the same projectedarea as the particle and it is equal to 2√(A/π).

The steel should preferably contain 0.2-4 volume % VC, preferably 0.5-3volume % and more preferably 1.5-2.3 volume %.

The amount of M₆C and M₇C₃ should be restricted to 2 volume %,preferably 0.5 volume %, and more preferably 0.1 volume %, each.

The hardness of the steel can be adjusted by selecting a propercombination of the austenitizing time and temperature, the cooling rateexpressed as cooling time in the temperature interval from 800° C. to500° C. (t_(5/8)) as well as the tempering temperature. Generally, thesteel is tempered twice for two hours (2×2 h) in order to reduce theamount of retained austenite to less than 2 volume %.

The mechanical properties of the steel after hardening and tempering toa hardness of 55-57 HRC should preferably at least one of the followingrequirements:

Yield strength (Rp0.2): ≥1700 MPa, preferably ≥1725 MPa, more preferably≥1750 MPa.

Tensile strength (Rm): ≥1950 MPa, preferably ≥2050 MPa, more preferably≥2050 MPa, most preferably ≥2100 MPa.

Elongation (A5): ≥3%, preferably ≥4, more preferably ≥5%, mostpreferably ≥6%.

Reduction of area (Z): ≥5%, preferably ≥10, more preferably ≥15%, mostpreferably ≥20%.

EXAMPLE 1

Table 1 discloses the hardness in Rockwell C (HRC) as a function of thehardening parameters austenitizing time and temperature. It can be seenthat the hardness easily can be adjusted in the range from 49 to 61 HRC.The composition of the ESR ingot was as follows: C 0.71%, Si 0.22%, Mn0.46%, Cr 5.01%, Mo, 2.24%, V 1.62%, Al 0.007%.

TABLE 1 Hardness (HRC) in the hardened and tempered condition. For allsamples cooling in vacuum with t8/5 = 300 s and tempering 2 × 2 h. Aust.T Time (° C.) (min) 540° C. 560° C. 580° C. 600° C. 610° C. 1050 30 57.356.2 54.9 52.4 48.9 1100 30 59.1 58.1 57.5 54.5 52.0 1130 10 60.4 59.158.4 55.9 53.7 1150 10 61.2 61.0 59.6 56.7 54.8

The temper resistance was examined for the steel austenitized at 1130°C. and tempered at 580° C. and 600° C., respectively. The steel sampleswere subjected to heating at 600° C. for 10 hours. In the first case thehardness decreased from 58.4 HRC to 53.6 HRC and for the second samplethe hardness decreased from 55.9 HRC to 52.8 HRC. Hence, the loss inhardness was 4.8 HRC and 3.1 HRC, respectively.

These values can be compared with the corresponding values for the steelUNIMAX® mentioned in the beginning. A sample of said steel having thenominal composition C 0.5%, Si 0.2%, Mn 0.5%, Cr 5.0%, Mo 2.3% and V0.5% was prepared. The steel was hardened to 57.8 HRC by austenitizingat 1050° C. for 30 min, with t_(8/5)=300 s and tempering 2×2 h at 540°C. The initial hardness was 57.8 HRC and the hardness after 10 hours at600° C. was 49.4 HRC. Accordingly, the loss in hardness was 8.4 HRC forthe known steel. It can thus be concluded that the inventive steel has asuperior temper resistance as compared to the known steel.

The cleanliness of the inventive steel was examined with respect tomicro-slag according to ASTM E45-97, Method A, Plate I-r and the resultis given in Table 2.

TABLE 2 Cleanliness according to ASTM E45-97, Method A, Plate I-r. A A BB C C D D T H T H T H T H 0.5 0 0.5 1.0 0 0 0.5 1.0

EXAMPLE 2

The ESR ingot of example 1 was hot rolled to a diameter of 196 mm fromwhich three samples were taken in the LC2 direction and subjectedexamination for mechanical properties. This steel sample was hardened toa hardness of 56 HRC by austenitizing at 1050° C. for 30 minutes coolingin vacuum with t_(8/5)=300 seconds followed by tempering twice at 560°C. for 2 hours. The following mean value of the examination are givenbelow:

-   -   Yield strength (Rp0.2): 1761 MPa    -   Tensile strength (Rm): 2117 MPa    -   Elongation (A5): 7%    -   Reduction of area (Z): 26%

EXAMPLE 3

In this example an inventive steel was compared to a standard matrixsteel used or forging tools.

The alloys had the following compositions (in wt. %) was

Inventive steel Comparative steel C 0.7 0.5 Si 0.2 0.2 Mn 0.5 0.5 Cr 5.04.2 Mo 2.3 2.0 V 1.6 1.2 W 0.01 1.6 balance Fe and impurities.

The alloys were subjected to standard heat treatment, forging and softannealing to a hardness of about 300 HBW. Both steels were subjected tohardening and tempering by heating to 1100° C. for 30 minutes, quenchingand tempering two times at 540° C. during two hours (2×2 h). Thehardness of the inventive steel was 57 HRC and the hardness of thecomparative steel was 56 HRC. The wear resistance of the steel wasexamined by the Pin on Disk method using 800 mesh Al-oxide papers fromthe same batch. The wear loss of the inventive steel was found to be 178mg/min and that of the comparative steel was 219 mg/min.

A further sample of the inventive steel was prepared in order to obtainthe same hardness as the comparative steel. This was achieved by heatingto 1100° C. for 30 minutes and tempering 2×2 h at 540° C. The hardnesswas 56 HRC. As expected, the wear loss of this sample was somewhathigher (189 mg/min) as compared to the steel having a hardness of 57 HRCbut substantially lower than that of the comparative steel having thesame hardness.

EXAMPLE 4

Samples of a steel of the same composition as in example 1 were preparedfor welding tests. Solid blocks of the steel were milled to have a sharp90° inside corner, the samples were to two different hardeningtreatments. The first heat treatment consisted of austenitizing at 1050°C. for 30 minutes cooling in vacuum with t_(8/5)=300 seconds followed bytempering twice at 560° C. for 2 hours. The second heat treatmentdiffered therefrom in that the austenitizing was performed at 1130° C.for 10 minutes.

The samples were then TIG-welded at room temperature (RT), 80° C., 225°C. and 325° C. using 1.6 mm diameter rod with three different standardwelding consumables. The applicants own Caldie TIG and QRO 90 TIG aswell as UTP A 696 TIG from UTP Schweissmaterial GmbH.

Cracking was experienced at all temperatures with the consumable CaldieTIG. However, surprisingly it was found that the two other consumablescould be used to produce crack-free welding also at RT without cracking.Accordingly, the inventive steel possesses a surprisingly goodweldability.

The steel of the present invention is useful for hot work applicationswhere the tool is subjected to abrasive wear. In particular, the steelis suitable as a tool for hot forging, press hardening, die casting,high pressure die casting or hot extrusion.

1. A hot work tool steel for hot forging, press hardening, die castingor hot extrusion consisting in weight % (wt. %): C 0.65-0.85 Si0.03-0.8  Mn 0.1-1.8 Cr 4.5-6.6 Mo 1.8-3.5 V 1.3-2.3 Al ≤0.1 N ≤0.12 Ni≤1 W ≤0.5 Co ≤2 Cu ≤1 Nb ≤0.1 Ti ≤0.05 Zr ≤0.05 Ta ≤0.05 B ≤0.01 Ca≤0.01 Mg ≤0.01 REM ≤0.2, and balance Fe and impurities.


2. The hot work tool steel according to claim 1, wherein the steelfulfils at least one of the following requirements: C 0.65-0.8 Si0.05-0.6 Mn  0.2-0.8 Cr  4.5-5.6 Mo  2.0-2.5 V  1.5-1.9 Al ≤0.05 N ≤0.08Ni ≤0.5 W ≤0.5 Co ≤2 Cu ≤0.5 Nb ≤0.05 Ti ≤0.01 Zr ≤0.01 Ta ≤0.01 B≤0.006 Ca ≤0.005 Mg ≤0.005, and REM ≤0.1.


3. The hot work tool steel according to claim 1, wherein the steelfulfils at least one of the following requirements: C 0.65-0.75 Si0.15-0.5 Mn  0.4-0.5 Cr  4.9-5.1 Mo  2.2-2.3 V  1.5-1.7 Al ≤0.03 N ≤0.05Ni   0.25 W ≤0.2 Co ≤1 Cu ≤0.2 Nb ≤0.005 Ti ≤0.005 Zr ≤0.005 Ta ≤0.005,and REM ≤0.05.


4. The hot work tool steel according to claim 1, wherein the steelcomprises carbides having a size of ≥1 μm and fulfils at least one ofthe following requirements concerning the amounts of carbides in volume%: VC 0.2-4 M₆C ≤2, and M₇C₃ ≤2.


5. The hot work tool steel according to claim 4, wherein the steelcomprises carbides having a size of ≥1 μm and fulfils at least one ofthe following requirements concerning the amounts of carbides in volume%: VC 0.5-3 M₆C ≤0.5, and M₇C₃ ≤0.5.


6. The hot work tool steel according to claim 5, wherein the steelcomprises carbides having a size of ≥1 μm and fulfils at least one ofthe following requirements concerning the amounts of said carbides involume %: VC 1.5-2.3 M₆C ≤0.1, and M₇C₃ ≤0.1.


7. The hot work tool steel according to claim 1, wherein the steel afterhardening and tempering has a hardness of 55-57 HRC and wherein thesteel fulfils at least one of the following requirements: Rp0.2 ≥1750MPa Rm ≥2100 MPa A5 ≥6% Z ≥20%, and

a cleanliness fulfilling the following maximum requirements with respectto micro-slag according to ASTM E45-97, Method A, Plate I-r: A A B B C CD D T H T H T H T H 1.0 0 1.5 1.0 0 0 1.5 1.0.


8. The hot work tool steel according to claim 1, wherein the steelfulfils at least one of the following requirements: C  0.66-0.75 Si 0.15-0.25 Mo  2.2-2.3 V  1.52-1.68 Al 0.001-0.03 N ≤0.05 W ≤0.1 Co ≤1Cu ≤0.15 Nb ≤0.005 Ti ≤0.005, and Zr ≤0.005.


9. The hot work tool steel according to claim 1, wherein the steelfulfils at least one of the following requirements: C  0.66-0.75 Si 0.15-0.25 Mo  2.2-2.3 V  1.52-1.68 Al 0.005-0.03 N ≤0.05 W ≤0.1 Co ≤0.5Cu ≤0.15 Nb ≤0.005 Ti ≤0.005, and Zr ≤0.005.


10. The hot work tool steel according to claim 1, having a mean hardnessof ≤360 HBW, wherein the steel has a thickness of at least 100 mm andthe maximum deviation from the mean Brinell hardness value in thethickness direction measured in accordance with ASTM E10-01 is less than10%, and wherein the minimum distance of the center of the indentationfrom the edge of the specimen or edge of another indentation shall be atleast two and a half times the diameter of the indentation and themaximum distance shall be no more than 4 times the diameter of theindentation.
 11. Use of the hot work tool steel according to claim 1 asa tool for hot forging, press hardening, die casting, high pressure diecasting or hot extrusion.
 12. The hot work tool steel according to claim10, wherein the maximum deviation from the mean Brinell hardness valuein the thickness direction measured in accordance with ASTM E10-01 isless than 5%.